1. Field of the Invention
The present invention relates to optical filters, and to optical sources incorporating such filters. In particular it relates to tunable filters and sources suitable for us, for example, in optical networks employing wavelength division multiplexing (WDM).
2. Related Art
The use of WDM and wavelength routing in optical networks has been proposed to provide the increased bandwidth required, for example, for the provision of multimedia services. Tunable lasers and filters are essential components for the implementation of such systems and desirably should provide tuning ranges of around 40 nm to allow complete coverage of the erbium amplifier transmission window around 1550 nm, for example.
It has previously been proposed to use a distributed Bragg reflector laser (DBR) as a tunable source [Oeberg M et al., "Complete single mode wavelength coverage over 40 nm with a super structure grating DBR laser" 14th Semi. Laser Conf. Proceedings, 1994, pp32-33]. While a tuning range as broad as 40 nm has been claimed for such a device, it has the disadvantages of being complicated to fabricate and to control. Achieving the broad tuning range requires adjustment of control currents in both the superstructure grating (SSG) and in a phase control section of the laser.
Another approach is the use of a laser with a grating-assisted vertically-coupled filter (GAVCF). The paper by Kim I., et al., "Broadly tunable vertical-coupler filter tensile-strained InGaAs/InGaAsP multiple quantum well laser", Appl. Phys. Lett., 1994, 64, (21), pp2764-2766, discloses one such device. Although a tuning range of 70 nm is claimed, and only one control current is required, the device is not able to cover all wavelengths but hops between longitudinal modes of the laser cavity.